1. Field of the Invention
The invention pertains to a drive unit with a drive and an electric machine, the drive being equipped with a drive shaft and the electric machine with a stator and a rotor, the rotor being coaxial to the stator and in effective torque-transmitting connection with the drive shaft, where the stator and the rotor interact with each other across an air gap, and where the drive shaft causes the rotor to execute a wobbling motion.
2. Description of the Related Art
Drive units of this type are known from, for example, the area of motor vehicle technology, where an internal combustion engine is combined with an electric machine to form a hybrid drive or where the electric machine is provided as a starter-generator. The electric machine can operate as a motor for starting the internal combustion engine or serve alone or in cooperation with an internal combustion engine to drive the vehicle. When the electric machine is operating as a generator, it can supply power to the users of electricity in the vehicle or store electrical energy in a storage unit.
The stator of the electric machine is usually stationary; for example, it can be mounted on the housing of the internal combustible engine or on the transmission housing. The rotor is in effective connection with the crankshaft of the internal combustion engine and can be connected to the crankshaft either directly or indirectly by way of extension pieces. A design of this type is shown in, for example, U.S. Pat. No. 5,952,746.
One of the characteristics of a rotating electric machine is the presence of an air gap between the stationary stator and the rotor, which moves relative to the stator. This air gap allows the stator and the rotor to interact electromagnetically with each other. To obtain an electric machine of high efficiency, it is necessary for this air gap to be as narrow as possible without jeopardizing the freedom of the rotor to rotate; the gap must also be constant in the circumferential direction. Typical values for the air gaps of synchronous machines are in the range of 1-1.2 mm, and the air gaps of asynchronous machines are typically in the range of 0.5k-0.7 mm.
As a result of its periodic, pulse-like operation, the internal combustible engine causes the crankshaft to execute flexural vibrations, which, for example, can be expressed as the slewing of the end of the crankshaft projecting from the internal combustion engine. In the case of an in-line engine, for example, this slewing motion occurs primarily in the plane defined by the stroke of the pistons. In addition, as a result of manufacturing variations in the bearings of the crankshaft, the crankshaft can also have a slight offset. As part of the continuing development of internal combustion engines, the degree to which the combustion chamber is filled with a fuel-air mixture has been increasing. The resulting rise in the combustion pressures increases the mechanical power output but also leads to the situation that, unless structural countermeasures are taken, the crankshaft is subjected to increased flexural vibrations.
During the operation of an internal combustion engine, therefore, the problem occurs that the rotor of an electric machine in effective connection with the crankshaft executes a wobbling motion. The slewing motion of the rotor is therefore superimposed on the rotational motion of the rotor, and thus the coaxial relationship between the rotor and the stator is impaired. This means that the radial dimension of the air gap of the electric machine is subject to continuous fluctuation as the crankshaft turns, which results in a considerable loss of efficiency and to variations in the moment produced by the electric machine, whether it is operating as a motor or as a generator. In addition, there is the very real danger that, as a result of the narrowness of the air gap, the rotor and the stator can come into contact with each other, which would damage the electric machine or even cause it to fail.
The problem described above has been known for a long time. Because it has been impossible so far to avoid or significantly to reduce the cause of the wobbling motion, that is, the flexural vibrations of the crankshaft, by technical improvements, efforts to remedy the problem have concentrated on modifications to the design of the electric machine.
In DE 199 37 545, an electric machine installed in the drive train of a vehicle comprising an internal combustion engine is described, in which machine the stator is attached rigidly to the housing. To decouple the rotor from the rotational irregularities of the drive shaft of the internal combustion engine, it is proposed that the rotor be supported on a bearing flange by at least one bearing, the flange being attached rigidly to the housing, and that the connection of the rotor to the drive shaft be designed with some elasticity. As a result of this measure, the concentricity of the rotor and the stator is maintained even during the operation of the internal combustion engine. This variant solves the problem, but because of the introduction of a separate rotor bearing, it is quite complicated and expensive.
DE 199 08 450 discloses an electric machine of the general type indicated above which is designed to exclude damage, especially the damage resulting from resonance vibrations of the rotor. According to the invention, a slip ring is provided, which limits the relative movement between the rotor and the stator in the radial direction. The disadvantage of this proposed electric machine is that it still allows fluctuations in the size of the air gap during operation of the engine. Only in the extreme case will the slip ring, which is intended to undergo wear during use, offer protection against destruction. A sacrifice in the efficiency of the electric machine is tolerated. The need to inspect the slip ring and to replace it after a certain period of operation also means additional maintenance work.
Proceeding from the basis described above, the object of the invention is to improve the design of an electric machine of the general type in question so that it allows reliable operation even under the influence of the flexural vibrations of the drive shaft of a drive, without the need for an additional rotor bearing or the introduction of protective parts which undergo wear.
The object is achieved by designing the geometric course of at least one of the two surfaces forming the boundaries of the air gap, i.e., the surface of either the rotor or of stator in a cross section parallel to the drive shaft, so that it at least approximates the geometric slewing curve described by the wobbling motion of the rotor. When the rotor and/or the stator is designed in this way, the rotational and wobbling motion of the rotor has little or no effect on the dimension of the air gap. The air gap therefore remains essentially constant, which has an advantageous effect on the efficiency of the electric machine. It is therefore impossible for any damage to be caused by contact between the rotor and the stator.
In an advantageous embodiment of the invention, the surfaces of the rotor and stator forming the boundaries of the air gap are essentially parallel to each other in a cross section parallel to the drive shaft. Thus a homogeneous magnetic field of constant strength in the axial direction can develop in the air gap, which also has a favorable effect on the efficiency of the electric machine.
It is especially advantageous for the geometric course of the surface of the rotor and/or of the stator forming a boundary of the air gap to be described, in a cross section parallel to the drive shaft, by a second-order curve. A curve of this type very closely approximates the geometric slewing curve of the wobbling rotor and can be manufactured relatively easily by the use of computer-aided methods during the fabrication of the rotor and/or stator.
In cases where the geometric slewing curve has only a small degree of curvature, it is advantageous to design the surfaces of the rotor and the stator forming the boundaries of the air gap as straight lines in a cross section parallel to the crankshaft, these lines being slewed with respect to the crankshaft. This also offers the advantage that the surfaces can be produced very easily.
It can be advantageous to produce the stator of the electric machine out of a stack of differently shaped individual plates. In this way, the course of the geometric slewing curve of the rotor can be transferred very accurately to the contour of the stator.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.